Helmets, which are one item of safety equipment, are now being worn proactively in numerous fields, including the industrial field, distribution field, construction and engineering field, and in the field of products used in social life. Among the materials for these helmets, polycarbonate (PC), acrylic-butadiene-styrene resin (ABS) and glass fiber-reinforced thermosetting resins (FRP) are widely known (Patent Document 1: JP 2003-105620 A). Among these, polycarbonate is attracting particular attention in terms of the properties mentioned below, and is starting to also be used widely in other technical fields.
Polycarbonate is a transparent amorphous resin. This resin exhibits minimal mold shrinkage, good dimensional precision and minimal water absorption, and therefore displays good dimensional characteristics. It is a material that has outstanding impact resistance, together with minimal creep and favorable heat resistance, electrical properties, self-extinguishing properties and light resistance.
A problem that has been identified for this material is its lack of chemical resistance (Non-Patent Document 1: “Plastic Data Book”, jointly edited by Asahi Kasei Amidas Corporation and the editorial department of “Plastics”, published Dec. 1, 1999 by Kogyo Chosakai Publishing Co., Ltd., pages 599 and 605).
In the case of helmets used outdoors, when the helmet is used under severe conditions, including under conditions of sunlight and high temperature, or even under conditions of low temperature and high humidity, contamination of the polycarbonate material that constitutes the helmet is severe, and in such cases, it is essential to perform a chemical treatment to remove surface contamination. It is thought that when this type of treatment is performed, the polycarbonate resin can undergo sudden damage, including dissolution or swelling, and whitening or cracking, which is problematic for its use as a product.
Table 1 illustrates the results of measurements performed by the inventors of the present invention to evaluate the chemical resistance of polycarbonate to a variety of different chemicals.
TABLE 1OrganicOxidizingChemical typesolventsSaltsAlkalisAcidsagents10-grade310176evaluation
The results in the table represent evaluation grades out of 10. A higher number indicates better chemical resistance. It is evident that polycarbonate is readily permeable to alkali and organic solvents, and does not exhibit stability to these chemicals. Further, because polycarbonate has ester bonds, it is thought that when exposed to alkaline solutions or hot water, the polycarbonate undergoes a hydrolysis reaction, resulting in degradation of the products using such materials. On the other hand, it has been assumed that even in the presence of water or acid, and under both normal temperature or low temperature conditions, almost no hydrolysis occurs. Under conditions of high temperature and high humidity, because hot water is present, the occurrence of a hydrolysis reaction is a concern.
When an alkaline solution is present, it is thought that the hydrolysis reaction can accelerate under the effects of the concentration and temperature of the solution. When used as the material of a product, these effects must be considered prior to use.
The following countermeasures have been proposed for the resin of polycarbonate helmets having the types of problems outlined above, but sudden damage to helmets during use have still been observed, meaning the uncertainty cannot be eliminated.
Polycarbonate resin molded bodies formed from laminates in which a polycarbonate resin is used as the base material, a water non-absorbing barrier layer composed of an ultraviolet-curable resin coating film is formed as a first layer on at least one surface of the base material, and an anti-fogging layer composed of a water-absorbing ultraviolet-curable resin coating film is formed as a second layer on the first layer are already known (Patent Document 2: JP 2007-210138 A).
The formation, on a plastic goggle lens or helmet shield composed of polycarbonate or the like, of a film that imparts physical properties such as good adhesion and superior wear resistance, and enables the inner surface to exhibit anti-fogging properties as a result of hydrophilicity, and the outer surface to exhibit waterproofness, water droplet resistance, oil resistance and stain resistance as a result of water repellency and oil repellency is already known (Patent Document 3: JP 2006-089859).
Multilayer articles comprising a substrate layer containing at least one thermoplastic polymer and fiber in a range between about 15% by weight and 75% by weight based on the total weight of the fiber-reinforced polymer substrate, and at least one top layer containing at least one thermoplastic polymer having structural units derived from at least one 1,3-dihydroxybenzene and at least one organic dicarboxylic acid are already known (Patent Document 4: JP 2008-500204 A).
Molded resin articles in which an abrasion-resistant organic hard coat layer having a haze of 5% or less is provided on one surface of a base, and an anti-fogging organic hard coat layer formed from a coating material described below is provided on the other surface are already known (Patent Document 5: JP 1996/041831 A1).
A composition comprising a polymer blend derived from: (a) a prepolymer comprising a component selected from the group consisting of free amine groups, free anhydride groups, and combinations thereof, and further comprising structural units derived from a dianhydride and a diamine, and (b) a polymer comprising a reactive component selected from the group consisting of structural groups, terminal groups, and combinations thereof, wherein the reactive component exhibits reactivity with the free anhydride groups, the free amine groups, or combinations thereof, and wherein the polymer blend is non-delaminated (Patent Document 6: JP 2010-51037 A).
Tests using polycarbonate alloys as a raw material have also been conducted. Alloys of polycarbonate and acrylic-butadiene-styrene resin, and polycarbonate and polyester or the like are mostly opaque, and cannot be used in technical fields that require transparency. The results of usage tests performed by the inventors of the present invention also revealed that the chemical resistance is unsatisfactory, and the conclusion was reached that the use of such alloys as a raw material for helmets and the like would be problematic.
The inventors of the present invention have worked hard in developing safety equipment such as helmets. Specifically, they have investigated not only the shape of helmets, but also the problems mentioned above relating to what materials are appropriate for use in helmets. They thought that the continued use of polycarbonate resin was essentially impossible. However, having viewed the large amount of research into the development of new resins that had already been conducted, they felt that the development of a novel resin that could be used as a material for helmets would be extremely difficult. Accordingly, they reached the conclusion that the most realistic approach was to resolve the above problems by using an existing resin, and developing a material that could be used in a stable state under usage conditions.
As described below, conventional polycyclohexylene dimethylene terephthalate copolyester resins have different properties from conventional polycarbonates in that they exhibit chemical resistance when used under normal conditions. However, the field in which the inventors of the present invention are proposing to use polycyclohexylene dimethylene terephthalate copolyester resins requires a material that is stable and has weather resistance and chemical resistance not in favorable environments such as that inside an indoor facility, but under natural environmental conditions, and similar or harsher usage conditions, which represent conditions in which these copolyester resins have not conventionally been used. The addition of other substances to alter the properties and make the resin capable of withstanding these types of conditions can be considered. Even considering the premise of inhibiting decomposition of the polycyclohexylene dimethylene terephthalate copolyester resin, the substance must have a dominant property that inhibits decomposition, and based on the results thereof, measures must be taken to address those factors not covered by the dominant factor. In any event, it was thought that modifying the resin to ensure that it is able to withstand usage conditions would be effective. Polycyclohexylene dimethylene terephthalate resins were investigated from this type of viewpoint.
Polycyclohexylene dimethylene terephthalate resins have been publicly disclosed (Patent Document 7: U.S. Pat. No. 2,901,466). Since then, polycyclohexylene dimethylene terephthalate copolyester resins produced by Eastman Chemical Company have also become well known (for example, Patent Document 8: JP H11-512484 A, and JP 3,432,830 B). Further, production methods have also been disclosed in U.S. Pat. No. 5,106,944 (Patent Document 9) and U.S. Pat. No. 5,668,243 (Patent Document 10). Modified polycyclohexylene dimethylene terephthalate copolyesters have been confirmed as having good impact resistance, minimal creep, and good heat resistance and electrical properties. In the formation of a copolyester, the reaction between a glycol component and a dicarboxylic acid component can be performed under normal polyester polymerization conditions. The copolyester is produced by a transesterification reaction. When a copolyester is produced from an ester-type dicarboxylic acid component, the reaction process can be composed of two steps. In the first step, a glycol component and a dicarboxylic acid component such as dimethyl isophthalate and dimethyl terephthalate are reacted under high temperature, typically about 180 to about 280° C., and under a pressure of about 0.0 to about 60 psig. The temperature of the transesterification reaction is preferably about 190 to about 240° C., and the pressure is preferably about 15 to about 40 psig. The reaction product is heated at an even higher temperature and under reduced pressure to form a polyester by a glycol elimination. The glycol is easily volatilized under these conditions, and is removed from the system. This polycondensation step which represents the second step can be continued under higher vacuum conditions, and typically at a temperature of about 240 to about 300° C., preferably about 245 to about 290° C., and most preferably within a range from about 250 to 270° C., until a polyester of the desired polymerization degree, as determined by I.V., is obtained. It is known that the polycondensation step can be performed under reduced pressure within a range from about 400 to about 0.1 mmHg (torr) (Patent Document 11: JP 2003-506592 A, Patent Document 12: JP 2002-523647 A). Further, a production method in which a polyester or an oligomer thereof is supplied and passed through a vertical stirred thin-film evaporator in a molten state, thus obtaining a polyester having a higher polymerization degree than that prior to supply is also known (Patent Document 13: JP 2000-309631 A).
As a result of active research, it is also known that polyesters such as poly-1,4-cyclohexylene dimethylene terephthalate-isophthalate and copolyesters thereof undergo hydrolysis in the presence of minute amounts of water, and innovations have been adopted to avoid this hydrolysis (Patent Document 14: JP 2007-285944 A).
A polyester support for an anti-fogging film formed from a biaxially stretched polyester film comprising, as a copolymerization component, a polyester of an aromatic dicarboxylic acid having a metal sulfonate group and a polyalkylene glycol is also known. An ultraviolet absorber is added to this polyester support for an anti-fogging film formed from a polycyclohexylene dimethylene terephthalate (Patent Document 15: JP 2004-359707 A).
Furthermore, the addition of a carbodiimide or the like to prevent hydrolysis of a polyester is also known (Patent Document 16: JP H11-506847 A).
The use of a film having an ultraviolet shielding effect during pattern transfer is also known (Patent Document 17: JP 4,105,919 B). Further, a thermoplastic polyester molding composition comprising a polyester resin and an ultraviolet stabilizing system, which is suitable for preparing a molded item that is resistant to decomposition and discoloration even when exposed to ultraviolet radiation for long periods is also known (Patent Document 18: JP H11-323100 A).
A polymer composition comprising a blend of at least two different polymers selected from the group consisting of polystyrene, polycarbonate, polyetherimide, polyolefin, polysulfone, polyethersulfone, polyacetal, nylon, polyester, polyphenylene sulfide, polyphenylene oxide and polyetheretherketone, and at least one elastomer having a tensile modulus less than about 50,000 prig for toughening these polymers (Patent Document 19: JP 3,647,036 B) is disclosed as having wear resistance, superior crack propagation resistance, lower yield strength, and increased transfer film ductility.